Journal of Ceramic Processing Research. Vol. 17, No. 4, pp. 300~303 (2016) 300 J O U R N A L O F Ceramic Processing Research Synthesis of Eu, Dy co-doped SrAl 2 O 4 phosphors by using liquid phase precursor process Yoon ah Roh † , Young Hyun Song † , Takaki Masaki and Dae Ho Yoon* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea SrAl 2 O 4 : Eu 2+ , Dy 3+ s are well-known as persistent luminescence phosphors with high brightness and long afterglow. They have been widely commercialized for afterglow applications such as security signs, emergency route signage, identification markers, and also bio-image in vivo. The persistent researches of SrAl 2 O 4 :Eu 2+ , Dy 3+ phosphor have been widely studied during the past decades. In this study, the properties of SrAl 2 O 4 : Eu 2+ , Dy 3+ phosphor were studied with wide range firing temperature and different annealing conditions. Synthesis of persistent phosphorescence in Eu 2+ and Dy 3+ ions co-doped SrAl 2 O 4 phosphors fired at 1100-1300 o C was conducted by using the cellulose assisted liquid phase precursor (LPP) process under reducing atmosphere. The SrAl 2 O 4 : Eu 2+ showed a typically broad emission band at 512 nm under excitation of UV range and Dy 3+ considerably enhanced the persistent luminescence intensity. Europium ions substituted for the two different Sr sites in the phosphor’s the monoclinic of host lattice resulting in greenish emission. The obtained phosphors were measured and analyzed by X-ray diffraction (XRD), scanning electron microscope (FE-SEM) and photoluminescence spectra (PL). Key words: Phosphor, LPP method, Photoluminescence. Introduction Phosphorescent materials have a great potential for various device applications and have been widely studied by many researchers. In the past, the ZnS-based phosphors had been extensively applied to many displays. However, the sulfide compound has a short durability because the compound is very unstable to moisture or carbon dioxide in the atmosphere. Hence these conventional sulfide phosphors are not sufficiently bright or long-lasting for actual application if radioisotopes are not added. Compared with the alkaline earth sulfides, alkaline earth aluminates are chemically stable in an ambient environment, and are used as new host materials in recent years [1]. Europium and dysprosium co-doped strontium aluminate (SrAl 2 O 4 :Eu and SrAl 2 O 4 : Eu, Dy) phosphors have been recrystallized to exhibit high brightness and long-lasting phosphorescence without radioactive materials, and available for a wide range of applications [2-4]. Since the discovery in 1996 of SrAl2O4 : Eu 2+ , Dy 3+ as a new persistent luminescent compound by Matsuzawa et al ., a number of researchers have developed techniques for their preparation, including sol-gel methods, hydrothermal synthesis, chemical precipitation, laser synthesis and solid state reaction. In previous reports, SrAl 2 O 4 : Eu 2+ , Dy 3+ phosphors were synthesized by high temperature solid-state reaction process, which requires quite long reaction time (4-10 hrs) in a high temperature (i.e., 1400- 1600 o C) [5-7]. As a result, the obtained particles have a large particle size which is impractical for printing applications, in which particles < 10 μm are required. On the other hands, liquid phase precursor (LPP) process which is the new synthesis method leads to achieve easily the deagrregated and nano-sized particles. This method is the low temperature synthesis caused by liquid medium such like sol-gel method. Unlike the sol gel process, however, this method can control distribution of the particles by using impregnation to cellulose homogeneously. We propose LPP process to achieve clear single phase of SrAl 2 O 4 :Eu 2+ , Dy 3+ phosphor at low temperature, and has high luminescence properties. Experimental Procedure Sr 1.7-x-y Al 2 O 4 : Eu x 2+ , Dy y 3+ (x = 0.8000, y = 0.0500) particles were synthesized using solution reaction. The raw materials strontium nitrate, aluminum nitrate, europium chloride and dysprosium nitrate were dissolved in deionized water, mixed solution were stirred for few minutes and then impregnated onto the cellulose precursor by 1 : 1 weight ratio. The sample was rapid fired at 600 o C to burn out the organics and raise the temperature to 1000 o C to form the SrAl 2 O 4 phase in an alumina crucible for 3 hrs. Afterward boron oxide is added as a flux to the obtained powders. Finally due to dope Eu 2+ selectively on SrAl 2 O 4 , the sample was fired at 1300 o C in H 2 /N 2 atmosphere and †These authors contributed equally. *Corresponding author: Tel : +82-31-290-7388 Fax: +82-31-290-7410 E-mail: [email protected]
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Journal of Ceramic Processing Research. Vol. 17, No. 4, pp. 300~303 (2016)
300
J O U R N A L O F
CeramicProcessing Research
Synthesis of Eu, Dy co-doped SrAl2O4 phosphors by using liquid phase precursor
process
Yoon ah Roh†, Young Hyun Song†, Takaki Masaki and Dae Ho Yoon*
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea
SrAl2O4: Eu2+, Dy3+s are well-known as persistent luminescence phosphors with high brightness and long afterglow. They havebeen widely commercialized for afterglow applications such as security signs, emergency route signage, identification markers,and also bio-image in vivo. The persistent researches of SrAl2O4:Eu2+, Dy3+ phosphor have been widely studied during the pastdecades. In this study, the properties of SrAl2O4: Eu2+, Dy3+ phosphor were studied with wide range firing temperature anddifferent annealing conditions. Synthesis of persistent phosphorescence in Eu2+ and Dy3+ ions co-doped SrAl2O4 phosphorsfired at 1100-1300 oC was conducted by using the cellulose assisted liquid phase precursor (LPP) process under reducingatmosphere. The SrAl2O4: Eu2+ showed a typically broad emission band at 512 nm under excitation of UV range and Dy3+
considerably enhanced the persistent luminescence intensity. Europium ions substituted for the two different Sr sites in thephosphor’s the monoclinic of host lattice resulting in greenish emission. The obtained phosphors were measured and analyzedby X-ray diffraction (XRD), scanning electron microscope (FE-SEM) and photoluminescence spectra (PL).
Synthesis of Eu, Dy co-doped SrAl2O4 phosphors by using liquid phase precursor process 301
this process improve their luminescence. A schematic
diagram of the experimental process were shown in
Fig. 1.
The synthesized powders were identified by an X-ray
diffractometer with Cu-Ká radiation. The scan speed
was 6 o/min and range covered was between 2θ = 20 o
and 90 o. The morphology of the powders was
observed by scanning electron microscopy (SEM, FE-
SEM XL-30, Philips). Photoluminescence spectrometer
with a pulse a Xe lamp. The luminescent properties of the
Eu2+-activated silicate phosphor were examined by
measuring the PL using a Darsa PRO 5300 PL system
(PSI Trading Co., Ltd., Korea) with xenon lamp (500 W).
The measurements were carried out at an excitation
wavelength of 390 nm. Excitation was performed with a
wavelength of 200-450 nm. The PLE spectra were
corrected by dividing the measured emission spectra by
the observed spectrum of a xenon lamp source.
Results and Discussion
In order to optimize the PL properties of a
SrAl2O4:Eu2+, Dy3+ phosphor, the firing temperature
was the variable considered. The phosphor was
synthesized at various temperatures ranging from 1100
to 1300 oC.
Fig. 2 shows the XRD patterns of SrAl2O4:Eu2+, Dy3+
with different temperatures and atmosphere. The
samples annealed at (a) 1100 oC, (b) 1200 oC and (c)
1300 oC under N2/H2 atmosphere, and another sample
(d) annealed 1300 oC under Ar-H2 atmosphere. The
SrAl2O4 crystal is known as a monoclinic lattice. In
case of (a), the annealing temperature is too low to
form a clear SrAl2O4 single phase. The secondary
phase, Sr4Al14O25, is observed which were attributed to
the characteristic lack of energy homogeneity during
the combustion. The SrAl2O4 phase appeared at lower
temperature in the strontium oxide-aluminum oxide
(SrO-Al2O3) system than in the SrAl2O4 phase. The
secondary phase was going to weak with increase of
the temperature. The samples fired at 1300 oC were
Fig. 1. Scheme of experiment process.
Fig. 2. XRD patterns of the Sr1.56Al2O4 : Eu0.8Dy0.06 phosphor firedat (a) 1100 oC, (b) 1200 o and (c) 1300 oC under H2/N2 reductionatmosphere and fired at (d) 1300 oC under H2/Ar atmosphere.
Fig. 3. SEM images and EDS analysis of Sr fired at (a) 1200 oC and 1300 oC under H2/N2 atmosphere; (c) 1300 oC under H2/Ar atmosphere(d) EDS analysis at the area of sample (b).
302 Yoon ah Roh, Young Hyun Song, Takaki Masaki and Dae Ho Yoon
formed completely clear single phase as shown Figs.
2(c) and 2(d). Therefore a reduction temperature range
was associated with the stability of forming single
phase.
The FE-SEM micrographs and EDS analysis results
shown in Figs. 3(a-d), illustrate the morphology and
particle size distribution of the SrAl2O4 : Eu2+, Dy3+. In
Figs. 3(a-b) it is seen that the SrAl2O4 : Eu2+, Dy3+
samples had the ball-like particles. With the reduced
temperature increasing, the grain growth was appeared.
But even when the sintering temperature reach
1300 oC, the average grain size is less than 100 nm,
suggesting that the luminescent powder is nanometer
grade while the average grain size of commercial
sample was is about 4000 nm. In addition, the sample
can be observed impurities on the particle surface, it
may be caused by Al-N bonding compounds as shown
EDS results (Fig. 3(d)). During reduction firing,
nitrogen gas which ruled reduction valance gas reacted
with samples. This reaction occur easily, because the
sample may contain the carbon could not be vaporized
during calcine the cellulose. The carbon has high
reactivity thus can aid the reaction with N2. A sample
fired at 1300 oC under H2/Ar reduction atmosphere was
obtained irregular shape particles such as a commercial
sample shown in Fig. 3(c).
The luminescence properties of SrAl2O4:Eu2+ were
discovered in 1968 [8-9]. Poort et al. [10] suggested
that the 520 nm emission band originate from the
4f65d1 → 4f7(8S7/2) transition of Eu2+ located at the two
different crystallographic strontium sites. The emission
spectrum at room temperature is shown in Fig. 4. It
displays a broad band with a maximum at 515 nm.
Leading to the green luminescence of the materials
under normal conditions. The intensity of sample fired
at 1300 o under H2/Ar atmosphere is higher than the
others including commercial sample. When the
temperature is 1300 o under H2/N2, a clear blue shift
occurs in the emission spectra of which the peak of the
emission spectra is 500 nm respectively as shown in
Fig. 4(c). The brightness is also greatly reduced, even
if it is same temperature with (b). There were two
reasons on this phenomenon. Firstly, Al-N bonding
leads to blue shift emission. Another reason is effect of
crystal field. According to the previous researches [11],
when the grain size reaches nanometer grade, the
Fig. 4. Emission spectra of the Sr1.56Al2O4 : Eu0.8Dy0.06 (a)commercial powders and samples fired at (b) 1200 oC and (c)1300 oC under H2/N2 reduction atmosphere and fired at (d) 1300 oCunder H2/Ar atmosphere. Besides is the atomic structure of AlO4
−.
Fig. 5. Excitation spectra of the Sr1.56Al2O4 : Eu0.8Dy0.06 (a) commercial powders and samples fired at (b) 1200 oC and (c) 1300 oC under H2/N2 reduction atmosphere and fired at (d) 1300 °C under H2/Ar atmosphere. Below is the scheme of mechanism of emission and trap system.
Synthesis of Eu, Dy co-doped SrAl2O4 phosphors by using liquid phase precursor process 303
luminescent materials shows the blue shift of emission
spectra. It leads to two phenomenon. One is to increase
the surface energy rapidly resulting in a distortion of
the atomic structure. In result crystal field strength of
surrounding Eu2+ ion is change. Another is ionic radii
of the atoms. The β-tridymite type monoclinic structure
of SrAl2O4 consists of AlO4 tetrahedral formed by a 3D
framework of oxygen ion surrounded by M2+ ions [12].
The means that smaller particle is that smaller crystal
field strength. The 4f electrons in Eu2+ are well
shielded by the outer shell, but the 5d electrons are
viable to splitting by the action of crystal field strength.
Since the excited 4f6 → 5d configurations of Eu2+ ion
is extremely sensitive to the change in the lattice
environment, the 5d electron may couple strongly to
the lattice [13]. Hence, the mixed states of 4f and 5d
configuration are splitted by the crystal field, which
may lead to the blue shift of its emission peak.
Fig. 5 shows the excitation spectra with different
reduction conditions. There were three excited levels.
Two shoulders are observed at 300 and 425 nm in
addition to the maximum at 370 nm on the excitation
spectrum of the commercial sample as shown in Fig.
5(a). The intense absorption of other samples fired at
1300 oC were observed at nearly same position shown
figure (b) and (c). These broad band absorption are
attributed to the parity allowed transition of Eu2+. The
phosphorescence mechanism of SrAl2O4 : Eu2+, Dy3+ is
involved in the direct excitation of Eu2+ due to the 4f
→ 5d transition occurs upon UV irradiation [25]. When